Servo mechanism



Dec, 16, 1947. J. H. PRATT ETAL 2,432,798

sERvo-MECHANISM Filed Nov. 24, 1943 3 Sheets-Sheet 1 Dec. 16, 1947. H, PRATT ET AL l 2,432,798

sERvo-MEGHANISM Filed Nov. 24, 1945 3 sheets-sheet 2 n afar-3 Dec. 16, 1947.

J. H. PRATT ETAL 2,432,798

SERVO-MECHANISM Filed NQV. 24, 1943 43 Sheets-Sheet 3 Patented Dec. 16, 1947 SERVO MECHANISM James Henry Pratt, Hall Green, Birmingham, and Albert Henry Godfrey Girling, Redgrave, Diss, Norfolk, England; said Gerling assignor to said Pratt Application November 24, 1943, Serial No. 511,630 In Great Britain November 24, 1942 (Cl. S-23) 2 Claims. 1

This invention has reference to improvements relating to servo-mechanisms and has for its primary, but not exclusive, object the provision of a compact and substantially foo-1 proof fluid operated servo-mechanism for actuating the brakes of relatively heavy vehicles.

According to the invention the improved uid operated servo-mechanism is characterised by a pumping unit, an accumulator unit into which the output from the pumping unit is delivered and means for disengaging the pumping unit from a driving agency when the required volume has been delivered into the accumulator unit.

An embodiment of the invention will now be described with particular reference to the accomH panying drawings which illustrate the invention in its application to a hydraulic servo-mechanism for operating the brakes of a heavy motor road vehicle,

In the drawings:

Figure l is a view of the improved hydraulic servo-mechanism drawn mainly in longitudinal vertical section together with such parts of a motor vehicle as are necessary to an understanding of the invention, the upper half of the section showing the positions occupied by the parts when the pumping unit is being driven and the lower half the positions occupied by the parts when the pumping unit has been disengaged from the driving agency.

Figure 2 is a sectional plan view of the servomechanism illustrated in Figure 1 and Figure 3 is a cross sectional View taken on the plane indicated by the line 3-3 of Fig. 1 looking in the direction of the arrows to said line.

According to the said illustrated embodiment of the invention the improved servo-mechanism incorporates a housing 5 a part 5a of Iwhich embraces the propeller shaft 6 which transmits the drive to the road wheels, not shown, said propeller shaft 6 being mounted in ball bearing races l carried at opposite ends of the part 5a of the housing 5. The housing 5 itself is supported by means of brackets 8 from a cross member 9 constituting part of the chassis of the vehicle.

The intermediate portion of the housing is divided into two sections by a transverse partition web 5b, said sections being termed henceforth the pumping section and the accumulator section.

The pumping section is provided with a longitudinally disposed cylinder Il] within which is reciprocably mounted an annular piston Il, hereinafter termed the pump piston II. This pump piston II is provided in the head with a bearing plug I2 which co-operates with an eccentric 6a formed on the propeller shaft 6 said pump piston I I co-operating with a spring I3 one end of which abuts the partition web 5b and the other end of which is seated in the bottom of a deep recess I Ia in the pump piston II so that the pump piston II tends to be moved towards the eccentric 6a by the spring I3.

The accumulator section is provided with a cylinder I4 within which is reciprocably mounted a piston I5 which will be termed hereinafter the accumulator piston I5, the space between the said accumulator piston I5 and the adjacent face of the partition web`5b constituting the accumulator I6. This accumulator piston I5 is secured to one end of a piston rod I'I the other end of which is located within the aperture in the central portion IIb of the pump piston I I. The last mentioned end of the piston rod I'I is provided with a flange Ila between which and a complementary flange IIc on the central portion IIb of the pump piston II is an annular rubber buffer I8.

Bolted to the housing 5 on the accumulator section side is the open end of a cylindrical container I9 hereinafter termed the reservoir I9. Within this reservoir I9 is disposed a pair of precompressed coil springs 20, 2i which at one end abut the closed end of the reservoir I9 and which at the other end abut a cup 22 the central portion of which tends to remain in contact with the end of the skirt of the accumulator piston I5, the adjacent end of the accumulator cylinder I5 constituting a stop for limiting the inward movement of the cup 22 under the action of the pressure of the said springs 20, 2 I.

The pump cylinder I0 is in communication with the reservoir IS by way of an inlet valve chamber 23 and a passage 5d in the partition web 5b. Disposed within the inlet valve chamber 23 is a pair of superposed inlet valves 24 which control the flow of liquid from the reservoir I9 to the pump cylinder I0, the duplication of the valves 24 being made as a safeguard.

The pump cylinder I0 is in communication with the accumulator I6 by way of a valve chamber 25 and passage 5e and the said valve chamber 25 has located therein a spring controlled one-way disc outlet valve 26.

The accumulator I6 is also in communication by way of the valve chamber 25 and passage 25a with a. second valve chamber 21 termed the balancing valve chamber 2'I when a valve 28a is lifted from its seating 21o, said balancing valve chamber 21 in turn being in communication by 3 a passage 21a with the pipelines, not shown, leading to hydraulic brake actuating means of known kind.

The passage 25a from the valve chamber 25 to the balancing valve chamber 21 enters at the top of the said chamber 21 and disposed within a depending housing 21h in the upper portion of the said chamber is a part-spherical valve 28a which co-operates with a seating 21C at the lower end o-f the depending housing 2lb. The partspherical valve 28a is carried at the upper end of a stem 28 the lower end of which carries a part-spherical valve 28h which co-operates with a seating 29a formed in the centre of a piston 29 hereinafter termed the operating piston 29, which is carried at the end of a plunger 29h reciprocably mounted in a guide 30 screwed into the lower end of the balancing valve chamber 21. The lower end of the said plunger 29h abuts a horn 3Ia carried by a lever 3I which is fulcrumed at one 'end to the housing 5 end of the said lever V3| being connected to a brake pedal, not shown, under the drivers control.

A coil spring 32 disposed within the balancing valve chamber 21 tends to press the operating piston 29 downwardly and thus draw the upper part-spherical valve 28a on to its seating 21e.

The lower part-spherical valve 28h is connected to the operating piston 29 by means of a cross bar v33 the intermediate portion whereof passes through a slot 28e in the valve 28h which is of a greater depth than the cross bar 33 thus permitting of a degree of lost motion. The two partspherical valves 28a, 28h have a balancing action as will be described hereinafter.

A iilling neck f which enters the inlet valve chamber 23 is provided for filling the said chamber and the reservoir I9 with liquid preferably the liquid normally used in hydraulic braking mechanism said neck 5f being closed by a screw plug 5h.

The outlet valve chamber 25 and the upper end of the balancing valve chamber 21 are closed also by screw yplugs 25d and 21d respectively.

The acting surface of the pump piston II is made with area greater than the area .of the acting surface of the accumulator vpiston I5.

Hydraulic packings are provided throughout the unit where necessary.

The accumulator 4springs 20, 2`I are associated with a bolt 33 which in conjunction with the cup 22 maintains the springs 2B, 2'I compressed when the reservoir casing I 9 is removed.

When the accumulator is full the 'flange llc at the end of the central portion of the pump piston I I comes into Contact with the 'boss in the center of the partition web 5b.

The operating piston 29 is .provided 4in the head with straight through passages 29h which open into the lower end of the balancing valve chamber 21, said lower end of the balancing valve chamber 21 communicating withthe reservoir I9 by way of a passage '5g and the inlet valve chamber 23.

The operation of the invention in .the embodiment described isas follows:

The accumulator springs 2D, 2| .are arranged to develop a force of 3970 lbs. so that when the accumulator I6 is at its maximum capacity a pressure of 1000 lbs. .to the 'square inch is developed, the pressure .being 800 lbs. -to ythe square inch when the accumulator 'I6 is nearly at its minimum capacity.

. 'Initially the whole housing 5 'is filled with liquid and on a subsequent outward stroke of the pump piston I I liquid is drawn from the reservoir I 9 into the pump cylinder Ill by Way of the duplicated inlet valves 24. On the rotation of the propeller shaft 6 the eccentric 6a bears on the bearing plug I2 in the closed end of the pump piston Il whereby the said pump piston II is moved inwardly and caused to force liquid from the pump cylinder I0 into the accumulator I6 by way of the disc outlet valve 26 valve chamber 25 and passage 5e pressure thus being built up in the accumulator I6 against the resistance offered by the accumulator piston I5 acting under the influence of the coil springs 20, 2 I.

When the accumulator I6 is fully charged the bearing plug I2 is removed out of contact with the eccentric 6a due to the release of pressure stored in the rubber buffer I8. This pressure is stored by reason of the fact that owing to the difference in areas the accumulator piston I5 travels further than the pump piston I I and consequently the buffer is compressed, but when the accumulator IS is full the buifer I8 maintains a pressure in the pump cylinder I0 and as this pressure leaks past the pump piston II the energy stored in the buffer I 8 is released and thus draws the bearing plug I2 away from the eccentric thus disconnecting the unit from the driving agency.

When it is desired to apply the brakes, the brake pedal is depressed thereby turning the lever 3| associated therewith and causing the horn 3la to lift the plunger 29. The lifting of the plunger 29 in its rst stages causes the seating 29a to engage the lower balancing valve 2812 thus sealing the balancing valve chamber 21 from exhaust. Further movement of the plunger 29 raises the upper balancing valve 28a off its seating 21e. The raising of the upper rbalancing valve 28a permits liquid to pass from the accumulator I6 into the balancing Valve chamber 21 and thence to the brake applying means by -way of the passage 21a. As the balancing valve chamber 21 is then under pressure there is a reaction on the operating piston 29 so that if there is a tendency to an excess pressure in the said chamber 21 the piston 29 is moved downwards against the resistance of the eifort being applied to the brake pedal. Such a downward movement of the piston 29 would allow the upper balancing valve 28a to close on to its seating 21C, thus cutting off the supply of pressure from the accumulator IS. Any further downward movement 0f the lpiston 29, even if slight, causes t-he seat-ing 29a to become clear of the lower balancing valve 28h, thus allowing excess pressure in the lbalancing valve chamber to escape, until as soon as the Apressure in vthe balancing valve chamber 21 has dropped to a pressure proportional to the brake pedal effort the piston 29 rises again and causes the seating 29a to engage the lower balancing valve 28h, so that the pressure in the balancing valve chamber tends to remain always at its lower value. Thus `a balancing action is provided and the braking eiort exerted made proportionate to the force applied by the driver so that a sense of feel is introduced.

In the event of more liquid vbeing required for the performance of a braking operation due to wear of the brake shoe linings the loss of volume in the accumulator I6 enables the pump to vcome into operation and pass liquid to the pipe lines in addition to the liquid normally .tending .to pass thereinto from the accumulator I6.

Should an abnormal pressure be required to be applied due ksay yto oily .brakeshoes 4this .may be obtained 'by applying such pressure to the brake pedal that the operating piston 29 is caused to act on the uid column in the balancing valve chamber 21. This operation is rendered possible for the reason that when the accumulator IE is full, a further travel of the piston rod l1 is stopped and the accumulator springs 20, 2| are consequently fully compressed so that eXtra pressure on the pedal cannot increase the com-pression of the said springs 20, 2l but must act through the operating piston 29 to increase the pressure in the pipe lines.

The propeller shaft E makes one revolution for every two feet of road travel so that if required the requisite pressure is rapidly forthcoming even in emergency and abnormal conditions.

The application of full braking pressure requires only a small pedal travel thus reducing time lag.

A servo-mechanism in accordance with the invention is compact, substantially fool prooi` and readily adaptable to existing motor road vehicles.

The invention is not limited in its application to motor road vehicles as it may be employed with other hydraulically operated mechanism in which similar `conditions of working apply.

We claim:

1. A fluid-operated servo-mechanism comprising, in combination, a pump having a suction stroke and a pressure stroke, resilient means for causing `the pump to execute its suction stroke, constantly operating mechanism for causing the pump to execute its pressure stroke after each suction stroke, and mechanism adapted to be actuated by fluid discharged from the pump to apply force greater than the force supplied by said resilient means to impel the pump in the direction of its pressure stroke so as to prevent the execution of a further suction stroke, the latter mechanism including a resilient connection through which such force is applied, and the pump being connected to a point at which the pressure is lower than the pressure in the pump at the end of its pressure stroke, by means of a slow leak to permit the discharge of fluid to cause 6 the pump to be impelled further in the direction of its pressure stroke by the energy stored in said resilient connection whereby the pump is brought completely out of engagement with said constantly operating mechanism.

2. A fluid-operated servo-mechanism comprising, in combination, a pump having a suction stroke and a pressure stroke, resilient means for causing the pump to execute its suction stroke, constantly operating mechanism for causing the pump to execute its pressure stroke after each suction stroke, and hydraulic motion-multiplying mechanism adapted to be actuated by fluid discharged from the pump to apply force to impel the pump in the direction of its pressure stroke so as to prevent the execution of a further suction stroke, the latter mechanism including a resilient connection through which such force is applied and in which energy is stored by the multiplication of motion, and the pump being connected to a point at Which the pressure is lower than the pressure in the pump at the end of its pressure stroke, by means of a slow leak to permit the discharge 0f fluid to cause the pump to be impelled further in the direction of its pressure stroke by the energy stored in said resilient connection whereby the pump is brought completely out of engagement With said constantly operating mechamsm.

JAMES HENRY PRATT. ALBERT HENRY GODFREY GIRLING.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,568,035 Reynolds Dec. 29, 1925 1,461,456 Reynolds July 10, 1923 970,329 Heck Sept. 23, 1910 1,881,939 Purdy Oct. 11, 1932 

